1. Field of the Invention
The present invention pertains to filters employed in optical communication systems, and more particularly, to dielectric multilayer interference filters for optical communication systems and to components which incorporate such filters.
2. Prior Art
In optical communication systems, optical elements which incorporate a thin dielectric multilayer filter therein are conventionally known, such that the component filter of the optical element is interposed within an extremely small gap within the optical pathway of the optical communication system. As an example of such a device, FIG. 23 shows a conventional optical element in which a dielectric multilayer filter is interposed between the end of one optical fiber and that of another, as described by H. Yanagawa et al. ("Filter-Embedded Design and its Applications to Passive Components", IEEE J. Lightwave Technol., vol. LT-7, pp. 1646-53, 1989). FIG. 23(a) is a plan view of the optical element, and FIG. 23(b) is a vertical cross-sectional view of the optical element through the line A--A' shown in FIG. 23(a). In the drawings, the ends of an input optical fiber 1 and an output optical fiber 2 can be seen opposing one another with an intervening dielectric multilayer filter leaf 3 which is situated within a slot 4 formed in a support base 5. In addition to the above mentioned dielectric multilayer filter leaf 3, support base 5 also supports and fixes the ends of the above mentioned input optical fiber 1 and output optical fiber 2.
By utilizing a dielectric multilayer filter leaf 3 having suitable optical characteristics, and appropriately adjusting the angle between dielectric multilayer filter leaf 3 and the optical axis of the optical element as determined by the orientation of the above mentioned groove 4 in support base 5, it is possible to fabricate an optical element which transmits light at one wavelength, and which reflects, and hence blocks, light at another wavelength. In this way, for an input optical fiber 1 in which light propagating therein includes a component having a wavelength given by .lambda..sub.1 and a second component having a wavelength given by .lambda..sub.2, where light at wavelength .lambda..sub.1 is necessary for operation of the optical communication system and light at wavelength .lambda..sub.2 is of no use or possibly undesirable, the wavelength .lambda..sub.2 component can be selectively blocked by means of the optical element, whereas the wavelength .lambda..sub.1 component is transmitted on to the output optical fiber 2 to propagate therein. Under ordinary conditions, light that is reflected backwards at the dielectric multilayer filter leaf 3 and reenters the input optical fiber 1 escapes therefrom via the cladding layer thereof. This rearward reflection can be maximized by carefully choosing the angle formed between the plane in which the dielectric multilayer filter leaf 3 lies and the optical axis of the optical element. In view of the above discussion, it can be seen that this type of optical element effectively transforms output optical fiber 2 into a filtered input optical fiber.
In order to fabricate the above described type of optical element, first of all, an optical fiber is fixed in a groove provided in support base 5 using an optical adhesive agent so as to extend beyond either end of support base 5, the above mentioned groove being parallel to the optical axis of the optical element to be fabricated. After removal of a central portion of the optical fiber within the above mentioned groove, thereby creating input optical fiber 1 and output optical fiber 2 from the single starting optical fiber, a groove is formed in support base 5 which intersects with the optical axis of the optical element, and forms a predetermined angle therewith. The dielectric multilayer filter leaf 3 is then fixed in this groove, again using an optical adhesive agent. By forming input optical fiber 1 and output optical fiber 2 from a single starting optical fiber as described above, assuming that the groove provided therefor is linear, it can be appreciated that the optical axes of input optical fiber 1 and output optical fiber 2 will be aligned with one another, thereby eliminating the need for subsequent alignment procedures.
The optical element of FIG. 23 as described above consists of a single, linear optical pathway with an intervening dielectric multilayer filter. In addition to this kind of device, other types of optical elements are conventionally known which employ dielectric multilayer filters. Examples of optical elements to which the application of one or more dielectric multilayer filters have been considered include multiple pathway optically coupling-splitting devices, wherein two or more optical fibers are brought into close approximation with one another in a parallel arrangement over a portion of their lengths to form an optical coupling region, whereby wavelength dependent optical coupling is achieved. By carefully controlling the physical characteristics of the component optical fibers over the coupling region, for example, index of refraction, presence or absence of cladding, etc., as well as the spatial arrangement of the individual fibers over the coupling region, the wavelength dependent coupling ratio between a pair of component optical fibers in the coupling region can be adjusted, whereby a desired distribution of outgoing light over two or more output optical fibers can be effected, thereby achieving splitting of one or more input optical signals into different wavelength components thereof.
In FIG. 24, one example of the above described type of multiple pathway optically coupling-splitting device is shown, wherein two dielectric multilayer filters are incorporated. The optical element shown in FIG. 24 includes two input optical fibers 6, 7, two output optical fibers 8, 9, an optical coupling region 10, two dielectric multilayer filter leaves 11, 12, a groove 13 wherein dielectric multilayer filter leaves 11 and 12 are inserted, and a base support 14 whereby input optical fibers 6 and 7, output optical fibers 8 and 9 and dielectric multilayer filter leaves 11 and 12 are supported. As described above, the coupling ratio between component optical fibers in the coupling region of this kind of device is known to be wavelength dependent. Thus, when an optical signal having two different wavelength components is supplied to optical coupling portion 10 via input optical fiber 6, depending on the wavelength of each component, and on the wavelength dependent characteristics of optical coupling region 10, it is possible to effect a substantial separation of the two wavelength components, such that a large proportion of one wavelength component is output from the optical element via dielectric multilayer filter leaf 11 and output optical fiber 8, and a large proportion of the other wavelength component is output from the optical element via dielectric multilayer filter leaf 12 and output optical fiber 9. In other words, splitting of the incoming optical signal into individual wavelength components thereof is achieved. Because it is not practical to achieve complete separation of the individual wavelength components of the input optical signal by means of the coupling region alone, in the device shown in FIG. 24, dielectric multilayer filter leaf 11 is incorporated into one of the outgoing pathways, and dielectric multilayer filter leaf 12 is incorporated into the other, whereby the effectiveness of separation according to wavelength can be improved. In addition to the devices shown in FIGS. 23 and 24, many other applications exist for dielectric multilayer filters.
With the conventional devices described above, diffraction may occur in the groove in which the dielectric multilayer filter leaves are mounted, thereby leading to a broadening of the intensity distribution, and hence optical losses. It is possible, however, to limit such optical losses by limiting the thickness of the groove for the dielectric multilayer filter leaves, that is, the length of the optical pathway which lies in the groove. As an example, using an optical fiber of 10 .mu.m core diameter with a core-cladding refractive index of 0.3%, a groove thickness on the order of tens of .mu.m or less, it is possible to limit diffraction losses which occur at the interstice between the input optical fiber and the output optical fiber to on the order of 0.5 dB. From the above, it can be seen that a correspondingly thin dielectric multilayer filter leaf is necessary to limit diffraction losses.
As is shown in FIG. 25, conventional dielectric multilayer filters consist of a hard base layer 15 with an overlying dielectric multilayer 16. The base layer 15 must be transparent, and must possess smooth surfaces and sufficient mechanical strength. For this reason, optical flat plate glass is often employed, for example, synthetic quartz glass, BK-7, etc., with a thickness of 0.5 mm or greater. The dielectric multilayer 16 consists of alternating low refractive index layers 17 and high refractive index layers 18. In addition to alternating low refractive index layers 17 and high refractive index layers 18, the dielectric multilayer 16 may also incorporate one or more additional layers having an intermediate refractive index between each pair of low refractive index and high refractive index layers 17, 18. For each component layer making up the dielectric multilayer 16, the thickness thereof is great to the extent that wavelengths it must handle are long. Additionally, in order to provide the required spectral characteristics, a fairly large number of individual component layers is necessary. In the case of fiber optic communication systems, the wavelength of light employed therein is ordinarily in the range of from 1.31 to 1.55 .mu.m, for which reason, the thickness of the dielectric multilayer 16 may reach 10 .mu.m in order to provide suitable spectral characteristics. As mentioned previously, the base layer 15 initially has a thickness of 0.5 mm or greater. Accordingly, in order to provide a dielectric multilayer filter leaf having a thickness of on the order of tens of .mu.m or less, it is necessary to grind and polish the surface of the base layer 15 opposite the surface thereof adjacent to the dielectric multilayer 16 until the desired overall thickness is achieved, after which the filter sheet thus prepared is cut to produce individual dielectric multilayer filter leaves of the desired size and shape.
Unfortunately, two shortcomings are inherent to the above described type of conventional dielectric multilayer filter. The first difficulty is that the grinding and polishing of the base layer 15 required for their fabrication entails a tremendous amount of precision manual labor, for which reason these filters are exceedingly expensive. Also, due to the fact that the base layer must be extremely reduced thickness in order to provide a dielectric multilayer filter leaf having a thickness of on the order of tens of .mu.m or less, hence making this base layer exceedingly fragile.
The second shortcoming relates to the fact that conventional ion assist vapor deposition methods are employed so as to provide a dielectric multilayer with highly uniform wavelength dependent optical characteristics. As a result, there is a tendency for residual compression stress to be present in the dielectric multilayers after the ion assist vapor deposition processing which secondarily leads to warping and curling thereof, for which reason grinding and polishing to produce a sufficiently thin filter can not be carried out. Examination of the graph of FIG. 26 will help to clarify the above point. FIG. 26 displays actual measurements taken using a talystep from the upper surface of the dielectric multilayer of two dielectric multilayer filters. In this graph, the curve labeled A is from a dielectric multilayer filter which was subjected to ion assist processing, and the line labeled B is from a dielectric multilayer filter which was not subjected to ion assist processing, but rather, underwent conventional vapor deposition. Both the dielectric multilayer filter corresponding to curve A, and the dielectric multilayer filter corresponding to line B included a base layer of 0.5 mm thick BK-7 glass, and the dielectric multilayer of each had a thickness of approximately 10 .mu.m and was made up of alternating layers of TiO.sub.2 and SiO.sub.2 for the purpose of separating an input signal into a 1.3 .mu.m wavelength component and a 1.55 .mu.m wavelength component. As FIG. 26 shows, the dielectric multilayer filter corresponding to curve A developed marked curling with a large central protuberance.
Investigation of the effect on the spectral characteristics of each of the two above described dielectric multilayer filters of varying humidity at 0% and 100% was also carried out, yielding the results shown in the graph of FIG. 27. As can be seen, sample B showed a variation of up to 25 nm, whereas sample A had exceedingly stable characteristics in response to differing levels of humidity. This finding relates to the fact that sample B has a more porous microstructure than that of sample A, such that water is able to enter the filter resulting in changes of the refractive index thereof. From FIGS. 26 and 27, it can be seen that while dielectric multilayer filters manufactured using conventional vapor deposition are more resistant to warping secondary to internal residual stress, dielectric multilayer filters manufactured using ion assist processing are more resistant to water vapor, and hence, more stable under conditions of varying humidity.
Comparison of the results of grinding and polishing revealed that the base layers of sample A filters were very susceptible to fracture as they were reduced to smaller and smaller thicknesses, such that no filters without cracking were produced when the overall thickness of the filter was polished down to the required thickness of on the order of tens of .mu.m or less. Conversely, the B sample filters could be successfully reduced to a thickness of 20 .mu.m. Thus, only the sample B filters which are vulnerable to humidity induced variations in spectral characteristics can be reduced to a suitable thickness by conventional methods.
Because of the exceptional expense involved in producing dielectric multilayer filters by conventional methods which require reduction of thickness by grinding and polishing, the present inventors investigated methods whereby it is possible to produce such filters having the desired thickness from the onset without need of such treatment. Several types of dielectric multilayer filters were examined consisting of a plastic film base layer, over which a multilayer interference membrane is applied, such as those disclosed by Sugiyama et al. (Japanese Patent Application, First Publication Serial No. Sho-63-64003), and that of J. A. Dobrowolski et al. (Applied Optics, vol. 28, no. 14, pp. 2702). The filters of both of the above two cited references were had a multilayer interference membrane consisting of on the order of twenty individual layers, each individual layer approximately 0.2 .mu.m thick. The filters of the former reference are produced by the so-called roll application method and are stored in roll form after manufacture thereof. Such filters have a rather nonuniform thickness of the multilayer interference membrane, and the multilayer interference membrane is prone to cracking and peeling away from the base layer under storage conditions. In the case of the filter disclosed by Sugiyama et al., a polyester film base layer having a thickness of 100 .mu.m is employed in order to provide adequate mechanical strength. Thus it can be seen that neither of the above two previously described filter materials is adequate for the purposes of the present invention.
During research to develop filter having a multilayer structure overlying a suitably strong and physically stable plastic film, the present inventors investigated using a thin plastic film polyimide layer which is applied over a conventional base layer of glass or the like, followed by application of the multilayer structure using an ion assist method, after which the resulting product is stripped away from the underlying glass layer. The material for the thin plastic film polyimide layer was selected so as to provide a material which can be applied over the glass layer using conventional painting or other liquid application means, and which is exceptionally resistant to high temperatures which occur during vapor deposition processes. For this purpose, an attempt was made to use commercially available polyimide resin compositions which were then spin coated over a 0.5 mm thick layer of BK-7 glass, followed by application of alternating layers of TiO.sub.2 and SiO.sub.2 by an ion assist vapor deposition method. This resulted in a polyimide base layer which was suitably adherent to the overlying multilayer, however, the polyimide base layer was excessively adherent to the glass layer and could not be successfully stripped therefrom.
It was then attempted to apply the the polyimide base layer over a silicon layer which has a very smooth, even surface. In this case, the resulting polyimide base layer and overlying multilayer could be stripped from the silicon by gradually introducing the tip of a sharp blade therebetween, but the stripped optical filter material curled up and was very difficult to handle. The above described curling occurred with the multilayer at the convex aspect of the curled material, and was thought to be secondary to a difference in the thermal expansion coefficient of the polyimide layer and that of the multilayer. Thus, with the polyimide layer having a listed thermal expansion coefficient of 2.times.10.sup.-5 /C..degree., and the multilayer having a thermal expansion coefficient of on the order of from 0.4.times.10.sup.-5 /C..degree. to 0.5.times.10.sup.-5 /C..degree., after cooling from the temperature at which ion vapor deposition is carried out (approximately 200 C..degree.), the polyimide layer contracts to a greater degree than the multilayer. This interpretation was substantiated by the fact that the curled material flattened out again upon reheating.